Looking at the situation, here are the numbers I'm looking it.
Tenshi only uses Ame-No-Murakumo Slash Level 1, and it's damage formula is 70% ATK - 50% DEF, and is a Spirit elemental attack. For 6F, Tenshi has 3980 Attack, so 70% is 2786. Since it factors in 50% DEF, this means we'd need 5572 DEF to negate it completely, more or less(Not factoring in Spirit Resistance and RNG). Obviously we don't need to negate the damage completely, just enough to not get murdered. Unfortunately, a Momiji at level 44 with all level bonuses into Defense, Guardian Subclass, 128 Library Levels in DEF, and 2 48% and 1 50% DEF equips is only 4322. To be fair though, this is with post 6F items, so the values will clearly be much lower, especially since the average level around 6F would be about 25-30.
So to get around Tenshi, you'd pretty much need to grind heavily just to have a Defense stat that doesn't suck. Uniform of Exorcism would be a good equip since it gives a decent amount of DEF(20%) and also gives Spirit resistance(+24) along with some EVA. The other 2 sub-equips will need to be the best DEF items available, preferable ones that also give good HP. Main equip should be the Elixir of Mages from Hina. Guardian is a decent Subclass for the boost to HP and DEF, plus it's skill set, but unless you're high leveled or got a lot of Training Manuals from reloading boss fights, you might not be able to get the most important skills. I'll go to that in a bit. Personally, I see Healer being the better subclass just to have the survivability.
So skills, the DEF buff is a must, as is the Healing. I also think the SPD buff should be obtained as well so you can get your turns faster, but right there we already have 54 skill points needed. If we get Guardian, we'd want Initiative and Absolute Defensive Line, the former for a starting 32% DEF buff, and the latter for a 30% damage reduction since Byakuren would be the only party member on the field, or so I assume. Those 2 alone are 20 more Skill Points.. If we get Healer, we'd need Prayer of Recovery for the healing. Not sure on Level, or if Efficient Treatment should be obtained to boost it, but if we maxed both, then we'd need 15 and 12 Skill Points respectively.
TL:DR: Probably better to just throw the other characters in just to get past 6F Tenshi. The grind isn't probably not worth all the time spent.